1. Field of the Invention
An unloading valve for use in a hydraulically operated hitch lift system for use on an agricultural tractor is provided. The unloading valve allows flow from a source of fluid to an auxiliary distribution valve when pressure requirements of the hitch lift cylinder are satisfied.
2. Description of the Prior Art
The prior art devices known to the applicants use a check ball, cradled in a spring loaded positioning member to act, as an unloading valve. The check ball was urged against the seat thus closing the unloading valve. An unloading piston was positioned to unseat this check ball to allow flow past the ball. The unloading piston carried a double spring loaded movable plunger and necessary retention hardware.
A detrimental pressure spike, occasionally rupturing fluid delivery lines and oil coolers, was generated upstream of this check ball upon its closing. The double springs in the unloading piston attempted to dampen the rapid reseating of the check ball as it contacted the movable plunger.
As the hitch control valve is moved to a position allowing the pressure signal on the back side of the unloading piston to decay the check ball was forced toward its seat by the spring loaded positioning member. The check ball through contact with the movable plunger of the unloading piston, pushed the unloading piston back toward a retracted position. The problem with the prior art devices occurred as the ball moved closer to its seat. Back pressure, due to pump flow restrictions, would increase and drive the ball faster toward its seat. This increase in acceleration caused instability of the check ball. In order to counter this acceleration the double spring and movable plunger was provided in the unloading piston with the intended function of dampening a portion of this instability. However, the lack of metering ability of the check ball as it moved against the seat resulted in a large overshoot or spike in hydraulic pressure upstream of the unloading valve.
The unloading valve described here overcomes the detrimental generation of this pressure spike.